High current capacity semiconductor device package and lead frame with large area connection posts and modified outline

ABSTRACT

A lead frame for a high power semiconductor device die has three external lead conductors, the outer two of which are reentrantly bent outwardly from the center of the lead frame. When the lead frame is overmolded, the outer conductors are spaced from a central conductor by an increased creepage distance along the plastic surface of the housing. Further, the lead sequence of the exterior leads is gate, source, drain for a power MOSFET. The post area for wire bonding to the source post is enlarged to permit wire bonding with at least three bond wires. The external conductors can be downwardly bent to form a surface mount device. The cross-sectional area of the external conductors is substantially enlarged, although only a small enlargement of the circuit board hole is needed. The package outline has a long flat area centered over the main die area, with a tapered end surface which allows the package to pry open a mounting spring for surface mounting of the package.

RELATED APPLICATION

[0001] This application is a division of U.S. patent application Ser.No. 09/103,035, filed Jun. 23, 1998 entitled HIGH CURRENT CAPACITYSEMICONDUCTOR DEVICE PACKAGE AND LEAD FRAME WITH LARGE AREA CONNECTIONPOSTS AND MODIFIED OUTLINE which is a non-provisional of U.S.Provisional Application No. 60/084,224 filed May 5, 1988.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a high power semiconductor device leadframe and package containing enlarged connection posts and conductorswith a novel lead sequence and having an increased dielectric creepagedistance between adjacent leads and having a modified outline.

[0003] Semiconductor devices such as diodes, thyristors, MOSgateddevices such as MOSFETs, IGBTs and the like are commonly formed in asilicon semiconductor die containing the device junctions. The die havemetallized bottom drain or other power electrodes and have source andgate electrodes or other power electrodes on their upper surface. Thedie are mounted on enlarged conductive lead frame pads and the powerelectrodes on the upper die surface have connection wires which are wirebonded by plural wires from the conductive electrode area of the die toflat connection post areas which are in turn connected to the exteriorlead conductors of the lead frame. These exterior lead conductors extendthrough a molded housing which overmolds the lead frame and die. Thelead frame will contain a plurality of identical sections, for example,20 or more sections which are simultaneously processed to receiveseparate die and wire bonds and overmolding. The individual devices arethen separated after the molding process. The final device may have wellknown industry standard package outlines, for example, the well knownTO220 or TO247 package outlines.

[0004] Known package structures have a current capacity which is limitedby the number of parallel bonding wires which can connect the die powerelectrode, for example, the source electrode of a power MOSFET or thecathode of a diode to a corresponding lead frame post. It would bedesirable to arrange the lead frame so that an increased number ofparallel bonding wires can be used to reduce package resistance withoutincreasing the size of the package.

[0005] Known package structures, particularly for MOSgated devices suchas power MOSFETs also conventionally have parallel external leadconductors in a sequence of gate, drain and source. This causes an addedspacing between gate and source leads. It would be very useful to havethe gate and source leads adjacent to one another, while maximizing thesource post area. It would be further desirable to increase theconduction cross-sectional area of the source or other elongatedexternal leads.

[0006] In conventional molded housing packages the lead frame conductorsextend from the interior of the high dielectric housing to the areaexterior of the package. The “creepage” distance along the surface ofthe package is thus related to the external spacing of the externalconductors, and limits the maximum voltage which can be applied betweenthese leads. It would be desirable to increase the creepage distancealong the package surface at which the lead frame leads extend out ofthe package without increasing the size of the package.

[0007] Known package structures have lead conductors extending outwardlyfrom the lead frame and through the plastic housing surface. These leadconductors are contentionally rectangular, or V-shaped and are designedto fit into a metallized opening in a printed circuit board. Thecross-section of these conductors must be large enough to carry thedevice current without excessive heating. However, the diameter of theholes in the board is limited, because their spacing is determined bythe spacing of the device lead conductors and their conductive bushings.It would be desirable to increase the cross-sectional area of these leadconductors, without excessively increasing the diameter of the holes inthe circuit board which receives the conductor.

[0008] The present package outline has a relatively thick plastic volumewhich is joined to a thinner volume by a vertical rise. The thickerregion extends from the lead conductor edge of the package to thevertical rise which is located above a central region of the die. Thethinner volume extends to the end of the package which is opposite toits output lead conductor side. the vertical rise to the thicker regionof the package forms a 90° angle to the top surface of the thickerregion. The material within this sharp angle tends to accumulate bubblesin the plastic during molding which leads to device rejections andfailure.

[0009] Further, when devices of the above outline are to be surfacemounted on a support board and held in place by a cantilevered spring,the spring presses atop the surface of the thick plastic region.Consequently, spring pressure is applied at a location which is removedfrom over the center of the die. It would be desirable to have the pointof application of the spring pressure located over the center of the diewhich is mounted within the package. It would be further desirable tosimplify the mounting of the package under a cantilevered spring withoutrequiring special tools.

BRIEF DESCRIPTION OF THE INVENTION

[0010] In accordance with a first feature of the invention it isrecognized that the area of the drain post and gate post of the leadframe can be smaller than that of the source post and the area then madeavailable can be used to increase the source post area. This makes itpossible to use an increased number of bonding wires to bond the diesource electrode to the source post of the lead frame, thus increasingthe current carrying capacity of the package.

[0011] In accordance with another feature of the present invention, anovel lead frame structure is provided in which, for a MOSFET typepackage for example, the lead frame external lead sequence is changedfrom the prior art gate, drain, source, to a novel sequence of gate,source, drain. This new sequence improves the application of the deviceby reducing the spacing between gate and source connections, thusreducing the leakage inductance of the gate circuit. The novel newsequence further makes it possible to increase the area of the sourcepost and to decrease the area of the drain post (which has a very widearea bottom die connection area), and makes it possible to use anincreased number of bonding wires from the die source to the lead framesource pad to reduce package resistance without increasing the packagesize.

[0012] A further feature of the invention permits an outward (from thelead frame center) reentrant bend of the two outermost leads from thecentermost lead frame lead where the lead conductors exit from thepackage, without reducing lead conduction cross-section. This outwardreentrant bend increases the creepage distance between the outer leadsand center lead along the surface of the package insulation to increasethe breakdown voltage of the device. By an outward reentrant bend ismeant a bend which redirects an elongated conductor to a generallyperpendicular path away from the center of the package, and then againredirects the elongated conductor to a path which is parallel, butspaced from its original path.

[0013] A further feature of the invention comprises the increase incross-sectional area of the lead frame external conductors withoutrequiring a significant increase in board hole diameter. Thus, in afirst embodiment it was found that making the normally rectangularconductor more square in shape, that a larger area of copper conductorcan fit into the same diameter opening. Further, it was found that theuse of a slight chamfer of the edges of the rectangular conductor willincrease the total cross-section of the final conductor.

[0014] As a still further feature of the invention, the plastic packageoutline is modified so that it has a uniform thickness and flat topexterior surface extending from the lead conductor edge and atopsubstantially the full area of the interior die. The end wall of thepackage opposite to the lead conductors is then tapered down toward thepackage bottom and edge at an angle of about 45° to the vertical.

[0015] As a result of this novel structure, when the package is mountedby a cantilevered spring, the center of the spring force against the topof the package can be centered over the center of the die, which is themost efficient location for application of force to the surface mounteddevice.

[0016] Further, the use of the tapered end surface increases the anglebetween the end surface and top surface to about 135°, thus making iteasier to exclude bubbles from this top edge area of the die duringmolding.

[0017] Finally, the tapered end surface of the package, makes itpossible to use the package as a wedge, to press the package under theraised lip of a cantilevered spring and under the spring without needingspecial tools.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a top view of a novel package containing the novel leadframe of the invention.

[0019]FIG. 2 is a side view of the package of FIG. 1.

[0020]FIG. 3 is an end view of FIG. 1.

[0021]FIG. 4 is a top view of a single section of a lead frame which canbe encapsulated in the package of FIGS. 1, 2 and 3, using the novelincreased outwardly bent lead conductors to increase creepage distancealong the plastic housing surface, and using the known sequence of gate,drain and source lead conductors.

[0022]FIG. 5 is a top view of a second embodiment of a lead frame likethat of FIG. 4, with the novel lead conductor sequence of gate, sourceand drain and an increased area source post.

[0023]FIG. 5a is a cross-section of FIG. 5 taken across the sectionlines 5 a-5 a in FIG. 5.

[0024]FIG. 6 shows a third embodiment of the invention in which the leadframe section of FIG. 5 is modified for a diode die.

[0025]FIG. 7 shows a fourth embodiment of the invention and is amodification of FIG. 4 to increase the source post area.

[0026]FIG. 8 is a cross-section of FIG. 7, taken across section line 8-8of FIG. 7.

[0027]FIG. 9 is a top view of a further embodiment of the invention,modified to be a surface mount type package.

[0028]FIG. 10 is a side view of FIG. 9.

[0029]FIG. 11 is an end view of FIG. 9.

[0030]FIG. 12 is a cross-section of a lead frame conductor of a standardTO220 package for a 50 ampere rating.

[0031]FIGS. 13 and 14 are cross sections of improved lead frameconductors made in accordance with the invention and having a highercurrent capacity.

[0032]FIG. 15 is a cross-section of an improved lead frame conductormade in accordance with the invention in which the edges of therectangular conductors are chamfered to enable the use of a largerconductor cross-section without an excessive increase in the circuitboard hole diameter.

[0033]FIG. 16 is a side view of the package outline of a prior artdevice with mounting spring.

[0034]FIG. 17 is a side view of a novel package outline in accordancewith the invention with a mounting spring.

[0035]FIG. 18 is a top view of the package of FIG. 17.

DETAILED DESCRIPTION OF THE DRAWINGS

[0036] Referring first to FIGS. 1, 2 and 3, there is shown a TO220 typesemiconductor device package 20 containing certain features of thepresent invention. Package 20 consists of a conventional plastic moldedhousing 21, which may be transfer molded, and which encapsulates asemiconductor die 22, shown in dotted lines in FIG. 2. A bottom mainconductive lead frame paddle 23 receives the bottom drain electrode ofdie 22. Three lead conductors 25, 26 and 27 extend through the frontwall 28 of plastic housing 21. Leads 25, 26 and 27 can be gate, drainand source contacts respectively for die 22 (if it is a power MOSFETdie), or, in accordance with an aspect of the invention to be described,can be gate, source and drain contacts respectively.

[0037] In accordance with an important feature of the invention,conductors 25 and 27 are bent away from the center of lead 26 and thepackage 20 at areas adjacent the front surface 28 by reentrantly shapedbends 30 and 31 respectively. This causes added creepage distances 32and 33 along surface 28 to permit the use of higher voltages betweenleads 25 and 27, and center lead 26.

[0038]FIG. 4 shows one section 35 of a multisection lead frame which canbe used in package 20 of FIGS. 1, 2 and 3. The shaded area 36 shows thesections of the lead frame which are conventionally trimmed away afterthe overmolding of housing 21 as shown in FIGS. 1 to 3. The paddlesection 23 may be of any suitable copper or copper alloy and may have athickness, for example, of 1.27 mm. The top surface of paddle 23 whichreceives semiconductor die 22 may be unplated to improve the bonding ofthe bottom electrode of die 22 (such as the bottom drain of a MOSFETdie) to the paddle 23 as by soldering or the like. The remainder of leadframe 35 may be nickel plated.

[0039] The portion of the frame 35 containing external lead conductorsmay also be nickel platted copper and may have a thickness of 0.8 mm.External lead conductors or fingers 25 and 27 terminate in solder posts40 and 42 respectively, corresponding to gate source connectorsrespectively. Note that solder posts 40 and 42 are coplanar and are in aplane which is above the plane of the upper surface of paddle 23. Paddle23 also has an extension 41 which is aligned with and is continuous withconductor 26. Thus, die 22 is provided at its top with a sourceelectrode 43 and a gate electrode 44. Gate electrode 44 is wire bondedto post 40 (which is suspended above and insulated from pad 23), andsource electrode 43 is wire bonded to post 42 which is also above andinsulated from pad 23 (see FIG. 2). Because of the relatively small areaof post 42, only two 20 mil bond wires 45 and 46 can be used to bondsource electrode 43 to post 42 (and thus external lead conductor orfinger 27 when the excess metal, shown shaded, is stripped away).

[0040]FIGS. 5 and 5a show another aspect of the invention which permitsthe use of additional bonding wires to the source electrode 43. Thus,the conductors 25, 26 and 27 are gate, source and drain electrodesrespectively, bringing the source and gate conductors closer together asshown in FIG. 5. Further, the source post 50 is now centrally locatedand is much wider than post 42 in FIG. 4. Significantly, post 50 may bewider than about one-half the width of the lead frame. This enables theuse of 4 20 mil bonding wires 51, 52, 53 and 54, each identical to wires45 and 46 in FIG. 4, from source electrode 43 to post 50 which extendsfrom conductor 26. The use of 4 bond wires instead of 2 permits asignificant reduction in package resistance and thus an increase in thecurrent capacity of the package without changing the package size or diesize.

[0041] The bonding wires 45, 46; and 51 to 54 are preferably purealuminum wires (0.9999 purity) and 20 mils in diameter. A single wirehas a resistance of almost 1 milliohm; two parallel wires have aresistance of about 0.5 milliohms, and 4 wires have a resistance ofabout 0.25 milliohms. Thus, the use of added wires causes a substantialreduction in package resistance.

[0042] The gate post 56, which receives only a thin gate bonding wire isreduced in area, and the pad extension 41 from pad 23 is moved to alignwith and to be connected to lead connector 27. Note that drain connector27 is at the potential of pad 23.

[0043]FIG. 6 shows the novel lead frame for a diode die 60, rather thana MOSFET die 22. Thus, in FIG. 6, a diode has a bottom electrode bondedto pad 23. The wire bond posts are modified to merge together posts 50and 56 of FIG. 5 into a single large area post 61. Post 61 enables theuse of 5 bonding wires 62, 63, 64, 65 and 66 from the diode topelectrode 67. The metal of the central conductor is stripped away duringthe metal stripping operation following plastic encapsulation.

[0044]FIGS. 7 and 8 show a further embodiment of a lead frame (twosections are shown), using the lead sequence of FIG. 4, in which thepost 42 of FIG. 4 is modified (enlarged) to allow 3 bonding wires tobond the upper electrode of a die on pad 23 to the post 70.Significantly, post 70 has a width which is close to about one-half thewidth of the lead frame. Note that pad 23 has a narrowed and curvedextension 71 connected to exterior conductor 26 (a drain conductor).

[0045]FIGS. 9, 10 and 11 show a modification of package 20 of FIGS. 1, 2and 3 to create a surface mount type package. Thus, plastic package 80is provided with a lead frame which, like those of the precedingfigures, has a main die support pad 23 and three output extending leadconductors 81, 82 and 83. Conductors 81, 82 and 83 may be gate, drainand source conductors respectively, and may have the basic structure ofthat of FIGS. 4 or 7. However, the leads 81 and 83 are reentrantly bentdownward as shown in FIGS. 10 and 11 to lie in a plane parallel to theplane of the exposed bottom surface of pad 23. Thus, the device of FIGS.9 to 11 is adapted for surface mounting, and otherwise has the variousadvantages previously described for FIGS. 4 and 7.

[0046]FIG. 12 shows a cross-section of any of the lead conductors 25 to27 of FIGS. 1 to 3. In the prior art TO220 package, the conductor 25 hada height of about 0.8 mm and a width of about 0.46 mm giving across-section area of about 0.388 mm². This was used for devices ratedat about 50 amperes RMS and required a printed circuit board hole ofabout 0.92 mm diameter.

[0047] It was found that the use of added bond wires within the package,as in FIG. 5, enabled an increase in device current. Thus, as shown inFIG. 13, the cross-section of contact 25 can be made 0.8 mm×0.8 mm for across-section of 0.64 mm². This requires only a minor increase in boardhole diameter to 1.15 mm, but permits an increased current capacity to65 amperes RMS without added temperature rise or increased packageresistance.

[0048] Further, as shown in FIG. 14, an increase of board hole diameterto 1.27 mm enable a contact cross-section of 0.8 mm×1.0 mm, increasingits current capacity to about 80 amperes RMS.

[0049]FIG. 15 shows the use of chamfered edges 90 on rectangularconductor 25. This permits the use of an increased copper cross-sectionof conductor 25 without a change in the thru-opening 91 in a circuitboard.

[0050]FIG. 16 shows the outline used for the plastic housing, such asthat of a TO 220 outline or the like. As in the case of the package ofFIGS. 1 to 10, the package 20 (shown with the lead frame of theinvention) has a relatively thick region with a flat shelf 200, arelatively thin region with a flat shelf 201 and vertical ornear-vertical walls 202 and 203. The wall 202 intersects top surface 200at almost 90°. this has created a molding problem in which air bubblesbecome entrapped in edge region 204. Furthermore, the package 20 isfrequently surface mounted atop a circuit board 210 or other surface bya cantilevered spring 211 which is fixed at one of its ends to thesupport surface 210. Spring 211 may have an upturned end edge 212, whichalso defines a pressure point to apply pressure to the top surface 200to press package 20 against the support surface.

[0051] In the prior art package design, the pressure point 215 isremoved toward an edge of die 22. It is preferable, in order to reducecontact resistance between frame 23 and surface 210 and to betterdistribute stress over the area of die 22, to move the point of pressureapplication toward the center of die 22.

[0052] A further problem with the package of FIG. 16 is that the edge ofthe package where wall 202 intersects surface 201 is low, and reducesthe tolerance in the bend of lead wire 46.

[0053] In accordance with the present invention, the package outline ischanged as shown particularly in FIGS. 17 and 18. Thus, in FIGS. 17 and18, the flat top surface 220 is extended well beyond the center of die22, and end wall 221 forms an angle of about 45° to the vertical.Consequently, the pressure point of spring 211 is applied over thecenter of die 22 to improve the mounting of package 20 to surface 210,both electrically and mechanically.

[0054] Furthermore, the novel wedge shape of the end of package 20 ofFIGS. 17 and 18 enable its mounting in spring 211, simply by pressingthe package 20 to the right in the figures, thus coming up the end 212of spring 211 and moving the package 20 under spring 211 to its desiredposition. A notch, or positioning posts not shown, can be formed in thepackage 20 to locate the package 20 in its desired end position.

[0055] A further advantage of the package outline of FIGS. 17 and 18 isthat the edge 230 will not entrap air during molding. Furthermore,additional room is provided for lead 46 and other similar leads withoutfear of their coming too close to a shallow shelf as in FIG. 16. Notethat in the package of FIGS. 1 to 10, that a short chamfer is found onthe edge equivalent to edge 202 in FIG. 16, to prevent the formation ofbubbles in that edge during molding.

[0056] Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A semiconductor device of increased currentcapacity without an increased size; said semiconductor device comprisinga lead frame, a semiconductor device die and a plurality of bondingwires; said lead frame comprising a thin generally flat conductive bodyhaving a die mounting pad and a plurality of parallel spaced externalconductors; at least one of said parallel spaced external conductorshaving an enlarged bond wire post at one end thereof which is adjacentto but is spaced from said die mounting pad; the bottom of said diebeing fixed to the top surface of said die mounting pad; a plurality ofsaid lead wires being bonded at one end thereof to the top surface ofsaid die and at the other end thereof to said enlarged bond wire post;said plurality of spaced external conductors comprising three equallylaterally spaced conductors which are disposed in the sequence of gate,source and drain conductors respectively for a power MOSFET and whereinsaid source conductor is the central conductor.